This invention relates to lithium cell's electrolytic solution containing lithium hexafluorophosphate, and a method for producing the electrolytic solution, and a method for purifying the electrolytic solution.
Hitherto, there have ben proposed various methods for producing lithium hexafluorophophate. For example, Japanese Patent Unexamined Publication JP-A-Sho-A-64-72901 discloses a method in which lithium fluoride which is in the form of solid is reacted with phosphorus pentafluoride which is in the form of gas, without using any solvent. In this method, after the reaction has been started, particles of lithium fluoride are coated with the reaction product (i.e., hexafluorophosphate). With this, the reaction does not proceed further, and thus the unreacted lithium fluoride is left in the reaction system. As another example, J. Chem. Soc. Part 4, 4408 (1963) discloses a method in which lithium fluoride dissolved in anhydrous hydrogen fluoride is reacted with phosphorus pentafluoride which is in the form of gas. It is, however, difficult to handle this reaction, beacause anhydrous hydrogen fluoride, which is high in vapor pressure, is used as a solvent. Furthermore, when the reaction product, lithium hexafluorophosphate, is taken in the form of crystals, an impurity, hydrogen fluoride, remains therein. If this impurity still remains in the electrolytic solution of a sithium cell, it interferes with the operation thereof. As mentioned hereinabove, the conventional methods for producing lithium hexafluorophosphate are not satisfactory in yield of the reaction, easiness to handle the reaction, and/or purity of the reaction product.
Lithium hexafluorophosphate is very stable, when it is ionically dissociated in a nonaqueous solvent. It is, however, very unstable and thus susceptible to hydrolysis and thelike during storage or transportation under a solid condition.
The above-mentioned lithium hexafluorophosphate and other lithium salts of fluorine-containing compounds are commonly used as electrolytes (solutes) dissolved in electrolytica solutions for lithium cells. These electrolytic solutions generally contain various acid impurities such as hydrogen fluoride. Of the electrolytes, lithium hexafluorophosphate and the like are easily decomposed by water contained in the electrolytic solution, thereby to produce acid impurities such as hydrogen fluiride, phosphoric acid, and oxygluorophosphoric acid. If an electrolytic solution containing acid impurities is used in a lithium cell, the acid impurities are reacted with the athode, the anode, and the solvent of teh electrolytic solution, thereby to lower the discharge capacity of the cell, to increase the internal resistance, to shorten lifetime of the cell, and to cause other problems. Hitherto, there have been various proposals for purging acid impurities from the electrolytic solution. For example, there have been proposals for purging water from the electrolytic solution in order to suppress the formation of acid impurities. However, it has been difficult to completely purge water therefrom. Thus, the electrolytic solutions purified by conventional purging methods were unsatisfactory in purity.